In semiconductor manufacturing technology, a mask (also referred to as a photomask or reticle) is used by a photolithography system to expose a pattern onto a substrate. The pattern can include many small or tight features, which are defined by the critical dimension (“CD”). CD defines such things as gate width, the minimum width of a line or the minimum space between two lines permitted in the fabrication of a device. As the critical dimensions of a mask or wafer become continually smaller (e.g., from 45 nm to 32 nm), it is important that dimensional and geometrical variations of the pattern features remain small. Such variations of the pattern features result in CD uniformity error (“CDU”).
Dimensional and geometrical variations of the pattern features can be introduced during various processes throughout mask fabrication. For example, while forming a pattern in a mask or semiconductor wafer, etching loading effects during an etching process may result in critical dimension variation associated with a global etching pattern density.
One solution that addresses reducing CDU errors introduced during an etching process is disclosed in U.S. Publication No. 2006-0000802 A1, entitled “Method and Apparatus for Photomask Plasma Etching,” issued to Kumar et al., which is hereby incorporated by reference. This solution provides a method and apparatus that reduces loading effects introduced in the plasma etching process by providing an ion-radical shield disposed in a process chamber above a substrate by a plurality of support legs. However, this solution presents several disadvantages: (1) the ion-radical shield may not be changed automatically or easily; (2) the support structure (plurality of legs) for the ion-radical shield results in the generation of particles during the etching process, particularly caused by vibrations; and (3) the ion-radical shield fails to remedy all mask critical density uniformity error, specifically global loading effects present from processes other than etching (i.e., baking, developing, and exposing process).
Accordingly, what is needed is a system and method for improving mask critical dimension uniformity.